Cell division cycle protein Cdc42 is a sub-class of the small G protein Rho GTPase family and is an important regulatory protein of many cell biological functions. First identified in Saccharomyces cerevisiae for its involvement in cell polarization, Cdc42 was then recognized to play important roles in cytoskeletal reorganization, cellular endocytosis, regulation of cell cycle and cell transcription. Activation of Cdc42, like that of most GTPases, is achieved through the exchange of guanosine-5′-diphosphate (GDP) for guanosine-5′-triphsopahte (GTP) binding. Cycles of activation and inactivation of Rho family of GTPases are regulated by three important class of proteins: guanine nucleotide exchange factor (GEF), which catalyzes the release of GDP for GTP binding; GTPase activating protein (GAP) as negative regulatory factor to accelerate the hydrolysis of GTP of Rho GTPases from the active to inactive state; and Guanosine nucleotide dissociation inhibitors (GDI) to prevent the separation of GDP from Rho GTPases, thereby inhibiting Rho GTPase activity.
Recent studies reveal abnormal activity of Cdc42 widely involved in the pathophysiology of human diseases including cancer and neurodegenerative diseases. Interestingly, Cdc42 gene mutations are not found in human tumors. Its alteration is mainly reflected in its abnormal form and overexpression and is dependent on the tissue microenvironment of disorders, closely related to tumor transformation and progression. As a key regulator of neuronal morphology, Cdc42 controls the fate of normal brain development. Cdc42 knockout mice do not live to birth and show significant brain malformations. Previous studies have shown that Cdc42 activates the epithelial to mesenchymal cell transition (EMT), playing important roles in intracellular transport and tumor cell invasion.
However, in three classical Rho GTPase family members, studies of Cdc42 lag far behind the RhoA and Rac1. This is partly due to fast activation/inactivation cycles of Cdc42, but also to the lack of selective small molecule research tools to help understand this process directly.
The Rho GTPase family proteins are involved in the signaling pathways that regulate a variety of biochemical and cellular functions, e. g. cell membrane and material transport, cell cycle regulation and cytoskeletal organization which is related to the control of cell morphology, cell motility and cell fate. In recent years, studies have shown that deregulated Rho GTPase signaling is involved in the pathogenesis of many diseases, and therefore it has become an important target for drug development.
Applications of small molecule modulators also contributed to the study of the functions of Rho GTPase family proteins. For example, the new types of effective small molecule compounds towards brain and cardiovascular system, fasudil and Y27632, target RhoA downstream effector signaling molecules and are recognized potent Rho/p160ROCK inhibitors. As a Rac1-selective inhibitor, NSC23766 in the recent study of the small molecule compounds targeting Rac1-GEF also greatly facilitate the understanding of Rac1 protein function. However there were almost no effective Cdc42 selective inhibitors. Secramine analogues of natural products galantamine recently showed to behave like a RhoGDI and inhibited Cdc42-dependent Golgi-mediated protein transport through cell membranes. Unlike the widely used Y27632 and NSC23766, the secramine study has been very limited. Cdc42 alteration is closely involved in tumorigenesis in many ways, including tumor transformation and metastasis: in addition, the development and maintenance of neurons is also heavily dependent on the normal Cdc42 activity.
NSC23766 was identified through a computer simulation of the structure of the compounds screened, and it fits the Rac1 molecular surface structure and the known Rac1 essential binding GEF (Cao, Y., J B Dickerson et al (2004) Rational design and characterization of a Rac GTPase-specific small molecule inhibitor. Proc Natl Acad Sci USA. 101 (20): 7618-7623). NSC23766 can inhibit serum or growth factor-induced Rac1 activation and Rac1 mediated lamellipodia formation. NSC23766 inhibits cell proliferation in human prostate cancer cell lines and tumor growth, and reduces cell invasion phenotype of the tumor cells which are dependent on the activity of endogenous Rac1. In addition, new research shows that NSC23766 treatment can improve Rac1-mediated spinal cord injury (SCI)-induced neuropathic pain (Tan, A M, S. Stamboulian, et al (2008). Neuropathic pain memory is maintained by Rac1-regulated dendritic spine remodeling after spinal cord injury. J Neurosci. 28 (49): 13173-13183).
Thus, there is a need to develop inhibitors of Cdc42.